So it is just a matter of time before they become standard equipment on all cars, right?

Automakers continue to search for a wide range of styling tricks to reduce aerodynamic drag in an effort to improve fuel economy.

General Motors global styling boss Ed Welburn, interviewed last week on the sidelines of the Frankfurt auto show, said “anything that helps reduce the interference along the side of a car is a good thing.”

So it looks like removable rear fender skirts are a done deal, right? Not so fast.

It turns out after extensive testing by GM, removable fender skirts flunked. They are more of a problem than a solution.

According to Welburn, there are several drawbacks:

 The fender skirt causes tire pressure to increase. Although he did not say it, neither GM nor any automaker wants to create a design that increases the probability of tire failure.

 Removable skirts are difficult to keep in place properly, plus there’s the added cost of each skirt.

 Pick one: To use a fender skirt, the rear tires need to be narrower to fit inside the wheel well or the vehicle track needs to be narrower or the body needs to be what he called “pulled out” to accommodate the same size tires that are steering the car.

Welburn said pulling the body out, essentially widening the body where the rear tires are located, increases aerodynamic drag.

While Welburn didn’t explain the other two choices described above, a narrower tire would reduce vehicle capacity and a narrower vehicle track could slightly increase the probability of rollover.

Says Welburn: "The short answer is, skirts don’t help on a production car."